$\displaystyle (9+9i)^3$

I tried multipling out the number and then arctan($\displaystyle \frac{y}{x}) $but its not right

I also tried finding the $\displaystyle \theta$ and dividing it by n, "3" ,but it is not the right answer.

it turns out to be $\displaystyle \frac{3\pi}{4}$